The present invention relates generally to vehicle transmissions and, in particular, to a vehicle transmission such as four wheel drive transfer case which includes an electronic control mechanism for clutch engagement.
Four wheel drive (4WD) systems for vehicles are becoming increasingly popular, due to the enhanced traction capabilities that such systems can provide in inclement weather or off highway conditions over conventional 2WD systems. Such 4WD systems generally include a torque transfer case which can either be integral with or separate from the main transmission, a rear output shaft connected to drive the vehicle rear wheel, a front output shaft connected to drive the vehicle front wheels, and means for drivingly connecting the input shaft to the front and rear output shafts.
In one type of 4WD system known as a "part-time" system, when in a 4WD mode, there is a direct drive connection between the front and rear output shafts of the transfer case. This direct drive connection does not permit the transfer case to accommodate different front and rear wheel speeds which can occur when turning the vehicle. Accordingly, the 4WD mode is generally used only on a "Part-time" basis, such as when lower friction road surface conditions, e.g., wet or snow covered pavement, may require increased traction capabilities. When turning the vehicle under these conditions, the lower friction road surface allows a limited amount of slip between the slightly faster spinning front wheels and the road surface. Otherwise, turning such a vehicle on a higher friction surface, such as dry pavement, results in what is known as "front wheel hop".
The transfer case also generally includes a shifting means operable by the vehicle driver for selecting whether the vehicle is to be operated in either 2WD or "part time" 4WD mode. Also, such shifting means often enables the driver to select between a "high range" 4WD mode and a "low range" 4WD mode. In the past, such shifting means typically included an all-mechanical arrangement wherein a driver operated shift lever was coupled to axially shift (via a shift fork) an internal shift sleeve in transfer case between the various operating modes. However, such systems require periodic lubrication and nevertheless exhibit a propensity to wear or lock up in time due to the constant physical interconnection between the plurality of moving parts. More recently, "part-time" 4WD transfer cases have been provided with electronic actuators for effecting the above mechanical shifting operations.
Some vehicles are provided with a "full time" 4WD system which is permanently engaged in all driving conditions. In this type of system, which has become known as an "all wheel drive" (AWD) system, the transfer case is generally provided with an interaxle differential for dividing torque between the vehicle front and rear differentials. The interaxle differential accommodates different front and rear wheel speeds when turning the vehicle, and thus enables the system to remain permanently engaged in all operating conditions. Also, in order to prevent excessive slipping between the front and rear wheels, and transfer cases typically include a selectively engageable clutch means which is operative to lock the interaxle differential upon sensing a predetermined slippage between the front and rear output shafts of the transfer case.